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Seismic rotation, curl of the seismic wavefield, represents a new observable which started beeing studied and analyzed namely in the past decade (Igel et al., 2012). Measurements of rotational ground motions are becoming relevant in a wide range of fields as it is documented also in two special issues devoted to advances in rotational seismology (Bull. Seis. Soc. Am, Vol 99, No. 2B, 2009, and J. Seismol, Vol 16, No. 4, 2012).

Rotaphone is a short-period mechanical sensor system designed to measure spatial ground motion gradients; it consists of geophones arranged in parallel pairs.
The spatial gradients are used to evaluate seismic rotational componets, i.e., components of curl of the ground velocity.
It is based on the patent CZ 301217 “Rotational Seismic Sensor System, Seismic Measuring Set Containing That System, and
Seismic Survey Method” Published in Věstník Úřadu průmyslového vlastnictví in 2009
No. 49/2009, B6, p. 5, G 01V 1/143